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Homework Help: What uncertainties should I assume when I only know the measured value

  1. Oct 5, 2006 #1
    I need to determine the propagation uncertainty of a calculated quanity (function of the variables mass, acceleration due to gravity, cap plate separation, and voltage), but I do not know the uncertainties of these measurements. I just know the values. What should I take the uncertainty to be? Should I just say +/- 0.5 or +/1 0.1 of the least significant digit? I missed the lab so I didn't actually perform the measurements, and the prof gave me this simulation program of the lab, but the simulation program says nothing about the uncertainties of the measurements.

    I am calculating charge from the following equation.

    q = (mgd)/v

    Examples of the values for the measured data are as follows...

    m = 1.x10x-15 kg
    v = 125 V
    d = 0.01 m
    g = 9.81 m/s^2

    What should I take the uncertainties to be??
  2. jcsd
  3. Oct 5, 2006 #2
    anyone? this is somewhat urgent.
  4. Oct 6, 2006 #3


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    Homework Helper

    That is the best you can do in such a circumstance - half of the least significant digit for each of the values - also for the gravitational acceleration. Just make sure about the amount of significant digits in each of the measurements because that will determine the magnitude of the uncertainty. Like your mass - a larger uncertainty will result from 1x10^-15 kg than from 1.0X10^-15 kg.
  5. Oct 6, 2006 #4
    well, the program shows 1x10^15 kg. I dunno. I will ask the prof tommorow I suppose.
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